@Infograph ask in the Cycles developer thread about E-Cycles, I try to give some information.
I don´t know how @bliblubli get the performance code wise but he use some AO tricks, different Cuda kernels and latest OIDN version 1.2, for example. Cycles use 1.0.
He use patches from different developers include them in E-Cycles long time before it lands in officially Blender release (if ever). Adaptive Sampling from Stefan Werner was created April 2019 but need one Year to land in Cycles.
Important for me is a easy to use render UI with presets, all in on place and so forth.
Same for denoising, one click and you get different node setups for OIDN.
Last features was Clay/AO render kernel which are really really fast for preview render and light groups where you get group from collections with lights and/or world environment again with one click.You can change lightning without rerender.
There is much more but I don´t like block of text and this is more as I am usually post.
EDIT: OIDN library was updated today to 1.2 and will land in Blender 2.90.